As the mystery deepened surrounding the new 787 Dreamliner battery system, U.S. transportation investigators were set to oversee more tests aimed at determining the cause of a fire central to the grounding of the Boeing jetliners.

The National Transportation Safety Board said the next round of testing was planned for Tuesday in Arizona.

Earlier tests conducted in Washington showed that a lithium-ion battery that caught fire aboard an empty Japan Airlines 787 in Boston this month was not overcharged, the board said on Sunday.

That determination ruled out one relatively simple explanation, potentially making the broader investigation more challenging and dragging out problems for the world's biggest aircraft manufacturer as well as the airlines globally that fly the technologically advanced wide body.

The Boston fire and a subsequent incident involving a battery alarm and a report of a burning smell on another 787 that made an emergency landing in Japan prompted the Federal Aviation Administration and other safety agencies globally to ground the Dreamliner last week.

Sunday's NTSB announcement "doesn't bode as well for a quick fix as Boeing would have liked," said John Goglia, a former member of the safety board, which investigates aviation and other transportation accidents.

"It's one step in the process. It's not great news, but it's not bad news either," Goglia said.

Mary Schiavo, a former U.S. Transportation Department inspector general who remains outspoken on aviation safety as a lawyer in private practice, agreed with Goglia.

"It does not sound like a quick resolution is in store for Boeing," the former watchdog said.

The main investigation, which also involves the FAA and Boeing, focuses on the 787's cutting-edge lithium ion battery system that powers the plane's auxiliary power unit. The APU, for instance, powers the plane's systems when the engines are idle.

The batteries in question are manufactured by Japan's GS Yuasa, under a subcontract to France-based Thales, Boeing said. But other companies, such as units of United Technologies and other vendors, contribute to the plane's extensive electrical system.

Officials from Japan's Land, Infrastructure, Transport and Tourism Ministry and the FAA conducted a site inspection of GS Yuasa's headquarters in Kyoto, Japan, on Monday and Tuesday, said Yasuo Ishii of Japan's Civil Aviation Bureau. The inspections are expected to last several days, Ishii said.

GS Yuasa said it is "fully cooperating in the investigation to determine the cause" of the problem but "could not comment on the details" of the inspection.

The investigation aims to identify the problem. It will be up to Boeing and its partners to figure out a remedy satisfactory to the FAA.

The next step for the safety board will occur on Tuesday in Tucson, Arizona, where the battery charger and power starter unit will be tested and relevant data downloaded. Testing on other components will be conducted by various manufacturers.

The examination of data from the Boston 787 indicated its APU battery did not appear to be overcharged by exceeding its designed voltage of 32 volts, according to Sunday's NTSB statement. But experts pointed out to CNN that there was no mention in the statement about how quickly the JAL 787 battery was discharging.

Discharging the battery too quickly, or with too low voltage, can also cause it to overheat, said University of Dayton professor Raul Ordonez, an aircraft electrical and computer engineer who spent time observing Dreamliner development at Boeing's Seattle headquarters.

Investigators in Washington have taken X-rays and CT scans of the lithium-ion battery that caught fire in Boston, the safety board said. They have also dismantled the battery and examined some of its individual cells.

The agency said it has also examined several other components from the plane, including wire bundles and battery management circuit boards.

"The fact that the NTSB is basically looking at every component around the battery, including the computer hardware and the (memory) software, means that they have no idea yet about a culprit and (they) suspect everything," Ordonez said.

Whatever the fix, Schiavo said any changes other than minor will require at least some re-engineering which will in turn require FAA approval. Both of those can result in a "slow process taking months, depending on the extent of engineering changes."

Boeing is using the lithium-ion batteries to electronically assist some of the functions that were previously performed using hydraulics. A lighter plane is more fuel efficient, which is one of the 787's main selling points.

There is no need to drain lithium-ion batteries fully before recharging, meaning less maintenance, though they can catch fire if overcharged.

Airbus uses lithium-ion batteries to power some systems aboard its A350 airliners. A spokeswoman said in a statement to CNN that Airbus "will carefully study any recommendations that come out of the 787 investigation and evaluate whether they apply to the A350 XWB," a new airliner still being developed by Airbus.

"Boeing was very, very lucky there was no catastrophic event," said Schiavo. "But, the luckiest of all is Airbus ... now they can make the fixes [to the A350 XWB] without the public relations hit Boeing has taken."

Boeing said Friday it will not deliver any Dreamliners to its customers as it works with the FAA over the battery concerns.

"We need to get the bottom of this," said Goglia. "We need to get comfortable with flying these airplanes again."